i'm hopefully going to purchase a 155 very soon (only a tiny handful (?less than 6) of 155's in Australia!), and ICE is my other passion, so i am also interested to know. my GT is being built for competition, but if i have a 155, it would also have a none-too-shabby system too.
i presumed there were door-mounted woofers in the front bottom corners of each door. is this not the case? looks like a big vacant space waiting for this to happen! so if there's not, i would want to remove the door trims to ensure there's a suitable hole in the metal door to accommodate the back/magnets of the speakers, then add an mdf wood spacer ring, and cut a hole in the door trim and fit a grill.
given the cars have power windows/mirros, there is obvoiusly an existing wiring loom that can cater for new speakerwire. all do-able i would think.
dashtop looks ideal to mount some tweeters, as part of a 2-way set of components.
i too am an audiophile (in terms of car audio), so would settle for nothing less than 5.25" 2-way components, and preferably 6.5" (esp as i have a spare set of Dynaudio MW160 woofers). rear speakers would not be important and only cater for rear passengers (extremely rare for me, esp if i have a 155 as a second car).
presuming i end up with a 155, my first steps would include removing the front door trims/cards. so perhaps you should do the same and determine what can fit. and post photos!